The invention generally relates to semiconductor substrate processing systems and relates in particular to laser marking systems for semiconductor wafers.
Laser marking systems for semiconductor wafers may be employed, for example, in semiconductor wafer processing systems that provide for the relative positioning and control of one or more semiconductor wafers in at least an x direction and a y direction such that laser marking is typically performed in the x y plane, the laser impinging the wafer generally along a z direction. The laser marking system may remain stationary, therefore, while a wafer stage is moved in the x and y directions. The wafer may be about 200 mm to 300 mm in diameter.
As shown in FIGS. 1A and 1B, a semiconductor wafer 10 may include a front side 12 on which a plurality of circuits 14a, 14b, 14c and 14d may be formed. The wafer 10 may later be diced into a plurality of individual circuits 14 following fabrication of the circuit elements 16 and 18 on each circuit 14a-14d. Four such circuits are shown in FIG. 1B. The semiconductor wafer 10 may also include a notch 22 to facilitate the identification of the proper orientation of the wafer 10 within the processing equipment.
In certain applications, it is desirable to also add an identification marking on either the front side or a back side of each circuit. Such marking is typically performed using a laser, and this laser marking may be used to identify not only the circuit but also manufacturing information in connection with each circuit, orientation information regarding the circuit, or performance data regarding the circuit.
In applications in which the front side 12 of the semiconductor wafer 10 includes circuits 14 that are very dense with elements and conductor paths, it is sometimes desirable to provide identification markings on the back side of the semiconductor wafer. For example, FIG. 2A shows the back side 16 of the semiconductor wafer 10, and FIG. 2B shows markings 24 on the back side of each circuit 14a-14d. The marking information may include a wide variety of text or other symbolic information, and is represented in FIG. 2B as a square 20 in the lower right corner of the front side of each circuit 14. Such a markings may be easily machine readable depending on the level of contrast of the marked versus un-marked regions.
The back side of most conventional semiconductor wafers, however, is typically ground to reduce the thickness of the semiconductor wafer so that thinner circuits 14 may be provided. Such grinding to reduce the thickness of the wafer is typically performed in a circular motion, and this causes a large number of very fine grooves 28, for example, in the general shape of a pinwheel to be formed on the surface of the back side 26 of the semiconductor wafer 10. This further complicates the automated detection of any indicia. In particular, incident light on the wafer in parallel with the grind pattern is generally reflected in a more specular manner than light incident on the wafer perpendicular to the grind pattern, which is generally reflected with stronger scattering. The orientation of the grinding grain varies radially and tangentially such that the grind marks appear as large spiral curvilinear patterns 28 across the wafer as shown in FIG. 2A and the grain orientation at each circuit 14a-d may vary as shown in FIG. 2B. The varying orientation and the characteristic pinwheel shape of these patterns are readily apparent by inspecting surface reflections in room lighting conditions. Grind patterns may vary from manufacturer to manufacturer, from lot to lot and from piece to piece. Other patterns with a varying grain direction may result from different grinding processes. The structure of the resulting grooves may be asymmetrical from side to side so that the scattering is asymmetrical from side to side.
One method of laser marking of the wafer 10 is to form a pattern (e.g., 30) into the surface of the back side 26 as shown in FIG. 3A. The depth of a pattern however, (as shown at d in FIG. 3A) may be so large, for example 10 mircons or more, that cracking of the semiconductor wafer may result from such high contrast laser marking. Patterns that avoid cracking may be less than about 10 microns deep for example in the range of 3 to 5 microns or less.
Another method of laser marking of the back side of a semiconductor wafer involves using a laser to provide a molten trace 32 on the back side surface 26, to thereby remove the relief surface provided by the grooves 28, as shown for example in FIG. 3B. Such a trace mark may have a very small depth of relief, of for example, 0 to 1.0 microns, and preferably about 0.5 microns. For example, U.S. Pat. No. 6,261,919, the disclosure of which is hereby incorporated by reference, discloses a system and method for forming a molten trace on the back side of a semiconductor wafer for purposes of marking. See also U.S. Published Patent Applications Nos. 2004/00600910, 2004/0031779 and 2004/0144760, the disclosures of which are hereby incorporated by reference, which each disclose high speed, laser-based marking systems and methods for Chip Scale Packages (CSP) for producing machine readable marks on workpieces and semiconductor devices with reduced subsurface damage produced thereby.
Such trace marks, therefore, may be made by changing the surface properties of the material, such as a ground silicon substrate, to form indicia. Marks formed may change absorption characteristics and reduce directional scattering effects by changing the surface texture, especially by flattening out the grind pattern. This reduced scattering effect may increase the fraction of specular reflections from the flattened area of the mark. At the same time increased absorption resulting from laser irradiation may reduce total reflections from the marked area. When fine indicia is formed, the feature sizes of the indicia may be similar to the scale of the features of the grinding marks. At the minimum laser marking power, the laser mark will start to appear as the grind mark begins to flatten. At higher powers, features of the grind mark will be further reduced and flattening will increase. At yet higher powers, material may be moved or removed increasing mark depth and spot to spot overlap marks may begin to form. Generally, incomplete flattening is an indication of a substantially minimum mark depth and spot overlap marks indicate a mark depth greater than the minimum.
The indicia laser marked by such marking may be any type of graphical mark, but are typically alphanumeric characters, pin indicators such as filled circles, circuit feature indication marks and chip orientation marks such as chevrons. Smaller die such as, for example, 0.1 by 0.2 mm die may be marked with a dot or orientation mark, while larger die, for example 2.5 mm by 20 mm may be marked with alpha numeric characters.
For CSP marking, the wafer is held in a wafer chuck that allows laser marking of indicia on the backside of die sites across the wafer. High accuracy marking is achieved by marking indicia in a scan field smaller than the size of the wafer, for example, over an 80 mm square field. To cover all of the sites to be marked with indicia on the back of the wafer, the wafer is stepped with a stage relative to marking field.
Although laser marking by altering portions of the relief surface provided by the grooves 28 generally imparts less stress on the wafer, the marks formed thereby are typically more difficult for a detection system to read. The typically very highly reflective nature of the back side of the wafer may make it further difficult for a detection system to read the laser markings for confirmation purposes. Further, the grooves on certain portions of the wafer may be nearly parallel with one another, providing highly directional reflections (such as grooves 28 resulting in pin-wheel type reflection images that rotate with angle of viewing). For this reason, such detection of markings on the back side is generally performed at very close range after each circuit has been diced from the wafer.
In certain applications, however, it would be desirable for a laser marking and wafer processing system to be able to perform inspection of laser markings on the back side of a wafer while the system is also inspecting the top side of the wafer. Correlating such top and bottom inspections provides highly accurate testing of the position of each marking on each circuit prior to dicing. Performing such a correlation requires a back side detection system that can read the back side of a wafer while it is being handled, as well as a front side detection system of the front side of a wafer. Reliable detection of the back side of a wafer, however, has proven to be extremely difficult due to the highly reflective and directionally reflective nature of the back side of the wafer due to the grinding process. Lighting from directly below the back side surface will be brightly reflected resulting in poor image contrast, and lighting from the side along the wafer may cast distractive unwanted highlights and shadows on the back side surface. Illuminating the highly reflective and directionally reflective back side surface for imaging from a distance, therefore, has not been satisfactory. Additionally, the interaction of the illumination with the varying backside grind pattern may create varying image quality across the wafer, and in some areas, image quality may be insufficient for successful image processing.
Further, in many wafer inspection applications, there are required working distance constraints. Wafers in CSP marking systems are typically held around the perimeter or by other areas on the back side of the wafer. Inspecting indicia at the edge of the wafer or at areas near chuck contact requires sufficient working distance from the illuminator and camera to the wafer to avoid mechanical interference or optical disruptions. Optical disruptions may include blocking portions of the illuminating field, light scattering off the chuck or associated mounting structures and onto the wafer, and blocking portions of the camera image field. Available space may be further limited by camera package size and by the imaging lens size.
In certain applications, it is desirable to inspect laser marked indicia on the ground back side of a wafer that is held in a chuck. Available illuminators designed for conventional wafer inspection do not provide images at sufficient working distance and with a uniform image quality for highly reliable inspection of wafer areas when different areas of the wafer are positioned for imaging relative to an inspection camera and illuminator.
There is a need, therefore, for a laser marking and processing system that may provide laser marking on the back side of a wafer, and that may provide automated correlation of the front and back sides of the wafer.
There is further a need for an improved wafer illumination system for inspecting laser marked indicia on the ground back side of a wafer when held in a back side marking chuck